Such a waveguide structure is frequently used, in conjunction with an antenna having a reflector confronted by a radiator or illuminator usually designed as a feed horn, for point-to-point communication via radio links, e.g. between an earth station and a satellite. With two pairs of carriers as noted above, the antenna can simultaneously handle signals on two outgoing and two incoming paths.
A waveguide structure designed to separate the several carriers from one another must provide low-loss coupling between a given carrier and a local signal channel, on the one hand, and must insure proper mutual decoupling of the carriers, on the other hand. Some systems satisfy these dual requirements by the use of separate feed horns and reflecting surfaces of the dichroic type. A more compact arrangement utilizes a single illuminator in cascade with frequency discriminators and polarization separators.
In U.S. Pat. No. 3,731,236 there has been disclosed a waveguide structure, referred to as a diplexer, with two coaxial but axially separated orthomode transducers of circular cross-section interconnected by four flat branch guides of rectangular cross-section which lie in two mutually perpendicular axial planes. The first transducer, having one extremity open to an impedance-matching transformer, has an extension of relatively small diameter at its opposite extremity which merges into a rectangular waveguide for one of two higher-frequency carriers, the other carrier in this frequency range passing through a rectangular waveguide extending laterally from this extremity. The second transducer is of relatively large diameter and also terminates in an axially extending and a laterally extending rectangular waveguide assigned to respective carriers in a lower frequency range.
A paper by R. W. Gruner titled "Compact Dual-Polarized Diplexers for 4/6 GHz Earth Station Applications", published on pages 341-344 of the 1977 Proceedings of the IEEE Symposium on Antenna Propagation, describes a structure with two coaxial circular waveguides for separating carriers of different frequencies, their mutual decoupling being improved with the aid of a corrugated waveguide section.
A problem encountered with rectangular branch guides extending radially from peripheral slots of a cylindrical guide member, designed to convey microwaves within not very distant lower-frequency and higher-frequency bands in the TE.sub.11 mode, lies in the tendency of such a branch to propagate not only carriers of the lower-frequency band in the fundamental TE.sub.10 mode but also carriers of the higher-frequency band in the higher-order TE.sub.20 mode. This results in a significant power loss of the higher-frequency signals which ought to bypass the branch guide in traveling to or from a different output or input port.